The majority of known and commercially available low-pressure discharge lamps are so-called compact fluorescent lamps. These lamps are determined to replace incandescent lamps used in a wide field of industry and home applications. Main advantages of these lamps are the relatively low power consumption and a long lifetime. Disadvantageous is however their relatively high price and large length dimension. Many configurations have been proposed to solve the length dimension problem. Such solutions include the multiple tube arrangements and the coiled tube arrangements. Most of these constructions however lack any means for protecting the lamp against adverse environmental effects.
U.S. Pat. No. 4,527,089 discloses a compact fluorescent lamp comprising multiple, individual tubes mechanically formed into an assembly and inserted into an outer envelope. The individual open-ended tubes are connected to each other through an arc directing means to form a continuous arc path. The outer envelope has a cylindrical shape, is hermetically sealed and includes an arc generating and sustaining medium such as an atmosphere of mercury and argon. The hermetic seal is mainly used in order to provide a securely closed container for the arc generating and sustaining medium. In case of any damage to the outer envelope, the arc generating and sustaining medium containing mercury is set free, which is harmful to the environment.
U.S. Pat. No. 6,064,155 discloses a fluorescent lamp with an outer envelope having an external shape of an incandescent lamp on a standard Edison-type base. The discharge tube is wound in a coil around the axis of the envelope and is disposed within the outer envelope. A ballast is arranged within the outer envelope, as well. A heat shield is disposed between the lamp and the ballast to thermally isolate the lamp from the ballast, whereby heat from the lamp will not adversely affect the ballast. Although this lamp is less harmful to the environment in case of damage of the outer envelope, difficulties may arise during production due to the use of the heat shield disposed between the lamp and the ballast. The manufacturing steps of mounting the ballast circuit board on the discharge tube and connecting the electrodes of the discharge tube to the ballast circuit appear to be in conflict with each other. On one side, if the ballast circuit board is already mounted on the discharge tube, the connection of the electrodes to the ballast is encumbered. On the other side, if the electrodes of the discharge tube are already connected to the ballast circuit, the insertion of the ballast circuit board is no more possible. In order to overcome these difficulties, long and therefore insulated connecting wires have to be used. A further shortcoming of this lamp is that the outer envelope is not sealed hermetically and therefore the discharge tube arrangement is not protected against adverse influence of the atmosphere. The discharge tubes are not fixed inside the outer envelope, they are only held by the electrodes.
Accordingly, there is a need for a compact fluorescent lamp configuration with an improved protection against dust and humidity, in order to make the lamp appropriate for use under extreme circumstances as well. There is also a need for an improved heat protection, which is easy to combine with the conventional manufacturing steps and therefore compatible with mass production. It is sought to provide a compact fluorescent lamp configuration, which readily supports different types of discharge tube configurations.